Travelling wave tube



NOV 12, 1957 G. DIEMER Erm.

TRIWELLINGl WAVE TUBE' Filed April 3. 1952 7374,17 m Am INVENTORS Hendrik Grqendijk Gesinus Dlemer Arie Versnel Kornelis Swier Knol Jphan Lodewi'k Hendrik Jonker United States Patent O TRAVELLING WAVE TUBE Gesinus Diemer, Kornelis Swier Knol, Johan Lodewijk Hendrik Jonker, Arie Versnel, and Hendrik Groendijk, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application April 3, 1952, Serial No. 280,214

Claims priority, application Netherlands May 11, 1951 3 Claims. (Cl. S15-3.5)

This invention relates to a travelling-wave tube.

In particular, the invention relates to a travelling-wave tube of the type comprising in combination, a conventional triode for ultrashort waves having a perforated anode and a helical conductor of a conventional travelling-wave tube.

Triodes for u1trashort waves, provided that they are well constructed, have the advantage of a low noise level but the disadvantages of low amplification and a relatively small band-width as compared to a travelling-wave tube. However, travelling-wave tubes have the disadvantage of a high noise level.

It is an object of the invention to provide a novel form of travelling-wave tube of the foregoing type having an extremely low noise level and good amplification for wide bandwidth signals.

This and further objects of the invention will be best understood from the following description.

According to the invention, a travelling wave tube comprises a cathode for generating a beam of electrons which is intensity modulated by a control electrode mounted adjacent the cathode. The latter comprises an annular member having a conically-bevelled aperture forming an angle of less than 120 and a plurality of grid wires supported by the annular member on a surface thereof adjacent the cathode. Spaced a given distance from the grid wires on the side of the annular member remote from the cathode is an anode which has an aperture of given diameter equal to or less than the distance between the anode and the grid wires and the depth of the aperture in the anode s equal to or less than half the diameter thereof. Beyond the anode is a helical conductor aligned coaxally with the electrodes in the triode portion of the tube and positioned as closely as possible to the anode along which an electro-magnetic wave to be ampliiied travels. The spacing between the control electrode and the anode is arranged so that, with suitable operating potentials applied thereto, the transit time of the electrons from the control electrode to the anode is less than the time of one full cycle of the electromagnetic wave which is to be amplied.

In order to minimize the noise level, the transit time should be as short as possible. Since the transit time is connected with the triode action of the first three electrodes, their spacing and relative potentials will determine the transit time of the electron beam and should be chosen for a minimum transit time.

in a particular embodiment of the invention, the helix may be conductively connected to the anode, in which event the anode-grid space in the triode portion of the tube must not be tuned to the frequency of the signal, but detuned so that anode is capacitatively connected to the grid so that the electron bunches produced by the action of the control electrode induces a voltage in the beginning of the helix. In another embodiment, the helix may be capacitatively connected to the anode and be coupled with the use of a loop in the anode-grid space.

ICC

By a suitable arrangement of the potential of the anode and of the helix, the alternating voltage on the helix obtained by the coupling with the anode-grid space will be amplified by the voltage induced in it by the electron bunches, so that the signal-to-noise ratio isas high as possible.

In a preferred embodiment, the cathode current is adjusted to be at least 10% of its saturation value. This results in a further improvement in the signal-to-noise ratio as explained in a co-pending application Serial No. 151,956, led March 25, 1950.

It is also desirable that the number of turns of the helix per wave-length and the potential applied to the anode in volts be adjusted so that the product of the former and the square root of the latter is at least 320, and furthermore, that the product of the number of turns of the helix per wave-length and the pitch of the helix is at least twice the diameter of the aperture in the anode.

The invention will nowbe described with reference to the accompanying drawing, in which:

Fig. l is a view, in cross-section, of one form of travel* ling wave tube in accordance with the invention;

Fig. 2 is a view similar to Fig. l of another form of travelling wave tube exemplifying the invention.

Referring to Fig. l, a travelling wave tube comprises a housing having vitreous wall portions 7, 8, 9, 10 supporting a plurality of metal discs 1, 2, 3 which, in turn, support respectively, a cathode 4, a control electrode 5 and an anode 6 in their respective operative positions. The cathode 4 is of the type disclosed in U. S. Patent 2,543,728, and is heated by a heating filament to a suitable temperature at which it emits electrons.

The control electrode 5, which serves to modulate the intensity of the beam of electrodes supplied by the cathode 4, comprises an annular member supporting a plurality of grid wires 14 on a surface thereof adjacent the cathode 4. The grid wires are spaced between 10 and 100 microns from the adjacent cathode surface and the annular member is provided with an aperture which is conically bevelled to form an angle qs (Fig. 1) equal to or less than The metal disc 3 supports an anode 6 spaced a given distance D from the grid wires 14 and providedwith an aperture having a diameter a and a depth d. The diameter a of the aperture of the anode 6 is adjusted to be equal to or less than the distance D while the depth d of the aperture is adjusted to be equal to or less than half the diameter a. The determination of the spacing D between the grid wires 14 and the anode 6 will be explained hereinafter.

A helical conductor 11, coaxally aligned with the cathode, the control electrode and the anode, has pitch S and is `connected at one end to the anode 6 and extends along the length of the tube. The helix is adjusted to be as close as possible to the anode while the free end of the helical conductor 11 is supported by a support electrode 12. A collecting electrode 13 is provided in the end of the tube for collecting the beam of electrons generated by the cathode and passing through the center of the helix.

In operation, an electromagnetic wave, which is the signal to be amplied by the tube, is introduced at the anode end of the helical conductor by any conventional means and travels along the helix of the conductor 11 to the supporting electrode 12 where it leaves the tube. The modulated electron beam tiowing through the center of the helical conductor 11 amples the signal in a conventional manner. Where the anode is directly connected to the helical conductor 11, the anode-grid space is not tuned to the incoming signal.

The spacing D between the anode 6 and the grid wires 14 is determined by the transit time of the electrons flowing therebetween7 and is arranged so that, with given potentials on the control electrode and the anode 6, the transit time of the electrons between the control electrode 5 and the anode 6 will be equal to or less than the time of one full cycle of the signal to be amplified, i. e. the frequency of the wave travelling along the helix. With the arrangement as described above, the signal-tonoise ratio is greatly improved.

To further improve the signal-to-noise ratio, the cathode current is adjusted to be equal to or more than 10% of its saturation value. Furthermore, the number of turns of the helix per wavelength of the incoming signal and the potential applied to the anode are adjusted so that the product of the former and the square root of the latter is equal to or more than 320 and that the product of the former and the pitch S of the helix is at least twice the diameter ofthe anode aperture.

Fig. 2 shows another form of travelling wave tube exemplifying the invention. ln this case, the helical conductor 11 is not directly connected to the anode 6 as shown in Fig. 1 but is capacitatively connected thereto. Referring to Fig. 2, the end of the conductor 11 extends through an aperture in the anode disc 3 and is shaped to form a loop 15 in the anode-grid space, the end of the loop being connected to a conducting plate 16. The conducting plate 16 is separated by a mica plate 17 from the disc 3, the combination of the disc 3 and the plates 16 and 17 serving to capacitatively couple the conductor 11 to the anode 6. A Icavity resonator comprising two annular curved members 19 separated by a mica plate 18 surrounds the anode-grid space and serves as the means by which the incoming signal is introduced into the tube.

While we have thus described our invention with speciiic examples and embodiments thereof, other modifications will be readily apparent to those skilled in the art without departing from the spirit and the scope of the invention as delined in the appended claims.

What we claim is:

l. A travelling wave tube comprising, in order and coaxially aligned with one another, a cathode, a control electrode, an anode having an aperture, and an elongated conductive helix for carrying an electromagnetic wave, said cathode generating an electron beam of given intensity, said control electrode comprising an annular member having a conically-beveled aperture forming an angle of less than 120 and a plurality of grid wires traversed by said electron beam and supported by said annular member on a surface thereof adjacent said cathode, means for applying an electrical signal to said control grid for intensity modulating said electron beam, said anode being spaced a given distance from said grid wires at which the transit time of electrons passing from said control electrode to said anode is less than the time of one full cycle of said electrical signal, the aperture in said anode having a diameter at most equal to said given distance and a depth at most equal to half its diameter, the end of said helix adjacent said anode being capacitatively coupled to said anode, and a cavity resonator surrounding the space between the control electrode and anode, said end of said helix forming a loop positioned in the space between said control electrode and anode and surrounded by said resonator and thus being inductively coupled thereto, whereby electromagnetic waves induced onto the helix by means of the loop and by means of the modulated beam are in additive relationship.

2. A travelling wave tube as set forth in claim 1 wherein the helix has a number of turns per wavelength of said electrical signal and a pitch at which the product of the former and latter is at least equal to twice the diameter of the aperture in said anode.

3. A travelling wave tube comprising, in order and coaxially aligned with one another, a cathode, a control electrode, an anode having an aperture, and an elongated conductive helix for carrying an electromagnetic wave, said cathode generating an electron beam of given intensity, said control electrode comprising an annular member having a conically-beveled aperture forming an angle of less than and a plurality of grid wires traversed by said electron beam and supported by said annular member on a surface thereof adjacent said cathode, means for applying an electrical signal to said control grid for intensity modulating said electron beam, and means for applying a predetermined voltage relative to the cathode to said anode, said anode being spaced a given distance from said grid wires at which the transit time of electrons passing from said control electrode to said anode is less than the time of one full cycle of said electrical signal, the aperture vin said anode having a diameter at most equal to said given distance and a depth at most equal to half its diameter, the end of said helix adjacent said anode being electrically coupled to said anode for inducing an electromagnetic wave thereon, lsaid helix having a given number of turns per wavelength of said electrical signal at which the product of the latter and the square root of vthe anode voltage is at least 320, whereby high ampliiication of the electrical signal with a low noise factor is obtained.

References Cited in the file ofv this patent UNITED STATES PATENTS 2,300,052y Lindenblad Oct. 27, 1942 2,367,332 Bondley Ian. 16, 1945 2,580,007 Dohler et al. Dec. 25, 1951 2,595,698 Peter May 6, 1952 2,602,148 Pierce July 1, 1952 2,608,668 Hines Aug. 26, 1952 2,632,130 Hull Mar. 17, 1953 2,681,951 Warnecke et al June 22, 1954 

